A hypothesis is proposed that life table, or more specifically reproductive longevity, of an organism is under genetic control and natural selection selects for a group of genotypes having the life table best fitted to a given environmental condition. This project is aimed to establish, with Drosophila, a formal genetic basis of factors, cytoplasmic as well as chromosomal, affecting life table and to obtain an experimental evidence supporting the above hypothesis. During the coming one year period, it is planned 1) to examine if the difference between reciprocal parental matings in reproductive longevity of F1 females and males occurs in general, or in certain specific combinations of strains and 2) to examine the relationship between temperature environmental condition and genetic systems regulating reproductive longevity.